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1. Mid-IR Emission Characteristics of Low-Phonon Erbium doped Ternary Chloride-based Single Crystals

   E. Brown, Z. D. (April 2021, Proc. SPIE 11682, Optical Components and Materials.)

   null (Ed.)

   The mid-infrared fluorescence properties of erbium (Er) doped low-phonon ternary chloride-based crystals (KPb2Cl5,Cs2HfCl6, CsPbCl3, CsCdCl3) have been investigated. All crystals were grown by vertical Bridgman technique. Following optical excitations at 805 nm and 660 nm, all Er3+ doped chlorides exhibited infrared emissions at ~2750, ~3500, and ~4500 nm at room temperature. The mid-infrared emission at 4500 nm originating from the 4I9/2 -> 4I11/2 transition showed long emission lifetime values of ~7.8 ms and ~11.6 ms for Er3+ doped Cs2HfCl6 and CsCdCl3 crystals, respectively. In comparison, Er3+ doped KPb2Cl5 and CsPbCl3 demonstrated shorter lifetimes of ~3 ms and ~1.8 ms, respectively. The temperature dependence of the 4I9/2 decay times was performed for Er3+ doped CsPbCl3 and CsCdCl3 crystals. We observed that the fluorescence lifetimes were nearly independent of the temperature, indicating a negligibly small non-radiative decay rate through multiphonon relaxation, as predicted by the energy gap law for low phonon energy hosts. The room temperature stimulated emission cross-sections for the 4I9/2 -> 4I11/2 transition were determined to be in a range of ~0.14-0.54 x 10-20 cm2 for the studied Er doped chloride crystals. 

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2. Spectroscopic Evaluation of Er: Ga2Ge5S13 Glass for Mid-IR Laser Applications

   E. Brown; Z. D. Fleischman; J. McKay; U. Hommerich; W. Palosz; S. Trivedi; and M. Dubinskii (July 2022, Optical materials express)

   We present mid-IR spectroscopic characterization of the low-phonon chalcogenide glass, Ga2Ge5S13 (GGS) doped with Er3+ ions. Under the excitation at ∼800 nm, Er3+:GGS exhibited broad mid-IR emission bands centered at ∼2.7, ∼3.5, and ∼4.5 µm at room temperature. The emission lifetime of the 4I9/2 level of Er3+ ions in GGS glass was found to be millisecond-long at room temperature. The measured fluorescence lifetimes were nearly independent of temperature, indicating negligibly small nonradiative decay rate for the 4I9/2 state, as can be expected for a low-maximum-phonon energy host. The transition line-strengths, radiative lifetimes, fluorescence branching ratios were calculated by using the Judd-Ofelt method. The peak stimulated emission cross-section of the 4I9/2 → 4I11/2 transition of Er3+ ion was determined to be ∼0.10×10−20 cm2 at room temperature. 

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3. Ho ³⁺ doped low-phonon single crystals and chalcogenide glasses for mid-IR source application

   https://doi.org/10.1364/OME.487911  

   Brown, Ei_Ei; Fleischman, Zackery_D; McKay, Jason; Merkle, Larry; Hommerich, Uwe; Palosz, Witold; Trivedi, Sudhir; Dubinskii, Mark (April 2023, Optical Materials Express)

   A comparative study was conducted to investigate the 3.9 µm mid-IR emission properties of Ho³⁺doped NaYF₄and CsCdCl₃crystals as well as Ho³⁺doped Ga₂Ge₅S₁₃glass. Following optical excitation at ∼890 nm, all the studied materials exhibited broad mid-IR emissions centered at ∼3.9 µm at room temperature. The mid-IR emission at 3.9 µm, originating from the⁵I₅→⁵I₆transition, showed long emission lifetime values of ∼16.5 ms and ∼1.61 ms for Ho³⁺doped CsCdCl₃crystal and Ga₂Ge₅S₁₃glass, respectively. Conversely, the Ho³⁺doped NaYF₄crystal exhibited a relatively short lifetime of ∼120 µs. Temperature dependent decay time measurements were performed for the⁵I₅excited state for all three samples. The results showed that the emission lifetimes of Ho³⁺:CsCdCl₃and Ho³⁺:Ga₂Ge₅S₁₃were nearly temperature independent over the range studied, while significant emission quenching of the⁵I₅level was observed in Ho³⁺:NaYF₄. The temperature dependence of the multi-phonon relaxation rate for 3.9 µm mid-IR emission in Ho³⁺:NaYF₄crystal was determined. The room temperature stimulated emission cross-sections for all three samples were calculated using the Füchtbauer-Landenburg equation. Furthermore, the results of Judd-Ofelt analysis are presented and discussed. 

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4. Comparative steady-state and time-resolved emission spectroscopy of Mn doped CsPbCl3 perovskites nanoparticles and bulk single crystals for photonic applications

   U. Hommerich, J. Barnett (April 2021, SPIE Proceedings , Optical Components and Materials)

   null (Ed.)

   Manganese doped inorganic halide perovskites continue to be of current interest for applications in light emitting devices and down-converters in solar cells. In this work we prepared Mn doped CsPbCl3 (Mn: CPC) bulk crystals and nanoparticles (NPs) and compared their emission properties. Bulk crystals were grown from the melt by vertical Bridgman technique and NPs were synthesized using a microwave assisted method. Under ultraviolet excitation at 350 nm, bulk crystal and NPs exhibited a broad orange emission centered in the ~600 nm range at room temperature. The broadbandemission was assigned to the intra-3d transition 4T1 → 6A1 of Mn2+ ions incorporated in the CPC host lattice. The Mn2+emission lifetimes were nearly exponential with values of 1.1 ms for NPs and 0.7 ms for the bulk crystal. NPs also showed exciton emission peaking at ~402 nm, whereas the bulk crystal exhibited no emission near the band-edge. Instead, the bulk material revealed a weak below-gap emission in the 450-550 nm region suggesting the existence of defect states. The excitation spectra for the orange Mn2+ emission from NPs and bulk crystals of Mn: CPC were significantly different indicating distinct excitation pathways. The excitation spectrum of the orange Mn2+ emission from NPs showed excitonic structure similar to the absorption spectrum suggesting an efficient energy transfer from excitons to Mn2+ ions. In contrast, UV excitation was less efficient for the bulk crystal and the excitation was dominated by below-gap excitation bands centered at 427 and 500 nm. 

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5. Crystal growth and characterization of undoped and Dy-doped TlPb2Br5 for infrared lasers and nuclear radiation detection

   Hömmerich, U.; Brown, E.; Kabir, A.; Hart, D.; Trivedi, S.B.; Jin, F.; Chen, H. (September 2017, Journal of crystal growth)

   We report results of the crystal growth and characterization of undoped and Dy-doped TlPb2Br5 for applications in infrared (IR) lasers and nuclear radiation detection. TlPb2Br5 (TPB) was synthesized from commercial starting materials of PbBr2 and TlBr and further purified through a combination of zone-refinement and directional solidification. For doping experiments, 2 wt% of DyBr3 was added to the purified TPB material. Crystal growth of TPB and Dy: TPB was carried out in a two-zone tube furnace by a vertical Bridgman method. Following optical excitation at ~1.36um, the Dy: TPB crystal exhibited efficient mid-IR emission bands centered at 2.87um and 4.35um with room-temperature lifetimes of 9.5 ms and 5.2 ms, respectively. The peak emission cross-sections were determined to be ~0.8x10-20 cm2 and ~0.5x10-20 cm2, respectively, which makes Dy: TPB a promising candidate for mid-IR laser applications. Besides its potential as a solid-state laser host, an undoped TPB crystal was also tested for gamma-ray detection. Using Cs-137 and Am-241 sources resulted in energy resolutions for gamma-rays as good as 1-2% (FWHM) at room-temperature under non-optimized conditions. 

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